DESCRIPTION

When you decode(=?encoding?X?ENCODED WORD?=), ENCODED WORD
is extracted and decoded for X encoding (B for Base64, Q for
Quoted-Printable). Then the decoded chunk is fed to
decode(encoding). So long as encoding is supported by
Encode, any source encoding is fine.
When you encode, it just encodes UTF-8 string with X encod-
ing then quoted with =?UTF-8?X?....?= . The parts that RFC
2047 forbids to encode are left as is and long lines are
folded within 76 bytes per line.

BUGS

It would be nice to support encoding to non-UTF8, such as
=?ISO-2022-JP? and =?ISO-8859-1?= but that makes the imple-
mentation too complicated. These days major mail agents all
support =?UTF-8? so I think it is just good enough.
Due to popular demand, 'MIME-Header-ISO_2022_JP' was intro-
duced by Makamaka. Thre are still too many MUAs especially
cellular phone handsets which does not grok UTF-8.